1. Field of the Invention
The invention relates to a nuclear resonance apparatus for examining a subject by nuclear magnetic resonance imaging having coils for applying fundamental and gradient fields to the examination subject and a radio-frequency means which irradiates the examination subject with a succession of radio-frequency pulses and which acquires the nuclear magnetic resonance signals emitted by the examination subject.
2. Description of the Prior Art
A nuclear magnetic resonance imaging system is described in German patent application No. P 31 35 335.5 which offers the possibility of deflecting the magnetic nuclear moments (nuclear spins) of an examination subject out of a preferred or equillibrium orientation with the assistance of a radio-frequency pulse, this preferred orientation being defined by a fundamental magnetic field. The frequency .omega. required for that purpose is defined via the equation .omega..sub.o =.gamma.B.sub.o, wherein .gamma. is the gyromagnetic ratio (characteristic value for the atomic nuclei under consideration) and B.sub.o is the magnetic flux density of the fundamental field. After the radio-frequency field is removed, the system of the nuclear spins strives to return to the initial condition while emitting nuclear magnetic resonance signals. The strength of the measured resonance signal is proportional to the number of excited nuclear spins and thus provides information about the spin density of the examination subject.
When the atomic nuclei whose resonance is observed are present in various chemical linkage conditions, different resonant frequencies occur on the basis of the different local magnetic fields at the location of these nuclei. In this case, the intensity relationships of the associated resonant lines indicate the relative frequency with which the nuclei under consideration are present in the various linkage conditions.
In order to obtain a nuclear magnetic resonant signal not from the entire examination subject but only from a specific, prescribed volume region, only the nuclear spins in the volume region of interest are excited. Selection of this region (slices are usually selected) is achieved by superposition of a suitable gradient magnetic field over the fundamental field. Given the employment of a selective radio-frequency pulse, the inhomogeneous magnetic field resulting therefrom (for example, linearly rising in the Z-direction) only allows the excitation of those nuclei wherein the magnetic field has the value required for the above resonance condition (in the example, a slice perpendicular to the gradient direction (Z-direction)). In order to distinguish the contributions of the various nuclei to the resonance signal within an excited slice, the gradient field perpendicular to the slice is switched off after excitation has ensued and a field gradient whose direction lies in the slice plane (for example, X-direction) is provided instead. As a consequence of the different magnetic field strengths, the nuclei of the excited slice precess at different speeds and therefore emit resonance signals having different frequencies. The spectrum S(.omega.) and thus the frequency components of the signal can be obtained from the resultant received signal S(t) by Fourier transformation (S(t).fwdarw.S(.omega.)). On the basis of this frequency spectrum S(.omega.), thus, the various contributions of the nuclei to the signal can be assigned to the location of their generation on the basis of their frequency. In the example, all nuclei in a strip perpendicular to the X-direction contribute to the overall signal with the same frequency because these experience the same strength of the magnetic field. A projection of the spin density onto the direction of the switched gradient is thus obtained.
In order to be able to construct an image of the excited layer, the dependency of the resonance signal on the third spatial direction (in the example, the Y-direction) must be identified. To this end,
(a) the above-described measurements can be repeated for may projection directions (suitable combinations of the X-gradient and Y-gradient) and a spin density image of the excited layer can be constructed from the acquired measured data by means of reverse projection, or
(b) the two-dimensional measuring matrix registered with the assistance of the specific combinations of the X-gradient fields and Y-gradient fields can be subjected to a two-dimensional Fourier transformation.
The amplitude of the nuclear magnetic resonance signal emitted by the nuclei under consideration depends not only on the spin density of the excited region but also, among other things, depends on the spin-grid relaxation, spin-spin relaxation, and field inhomogeneities. The influences of the above parameters can be imaged in a simplifying gyro model wherein the nuclear spins of the examination subject are symbolized by rotating magnetic moments (gyro) in a fundamental magnetic field. The fundamental field should thereby be oriented in Z-direction. In this model, the alternating magnetic field of the excitation pulse influencing the nuclear spins of the examination suject rotates these out of the Z-direction by a defined angle (flip angle). In the case of a 90.degree. pulse, all spins after the excitation lie within the X-Y plane in parallel alignment. The subsequent precessional motion of the magnetic moments is accompanied by
(a) the spin-spin relaxation (interaction of the spins with one another leads to a decrease of the magnetization components in the X-Y plane (cross-magnetization) with the time constant T.sub.2 in the framework of the above-described model);
(b) the spin-grid relaxation (interaction of the spins with the grid leads to a return of the magnetization into its initial condition parallel to the field with the time constant T.sub.1);
(c) field inhomogeneities (effect a divergence of the gyroscope spin relation to the X-Y plane, this gyroscope spin precessing with different frequencies for this reason).
In general, T.sub.1 &gt;T.sub.2 applies for the relaxation times T.sub.1 and T.sub.2. The amplitude of the signal (FID signal, free induction decay signal) emitted by the excited nuclei is defined, among other things, by the resulting magnetization component in the X-Y plane. Even more than the spin-spin relaxation, the dephasing of the spin caused by the field inhomogeneities effects the decrease of these components. The decrease in cross-magnetization caused by both processes is often described with the time constant T.sub.2 *.
Since the dephasing caused by field inhomogeneities has a rigidly prescribed time behaviour for every location, this process can be largely reversed by means of suitable measured (refocussing, for example by means of a .+-.180.degree. radio-frequency pulse).
An increase of the amplitude of the resonance signal to a maximum (spin echo) at which the dephasing caused by field inhomogeneity is largely compensated and, finally, a renewed decrease of the amplitude with the time constant T.sub.2 * due to the renewed dephasing can be observed. This focussing process can be repeatedly applied (for example by a sequence of .+-.180.degree. pulses) and leads to further echoes. The chronological decrease in the strength (maximal amplitudes) of these echoes therefore provides information regarding the relaxation constant T.sub.2.
The above spin-echo method uses a pulse sequence as shown in FIG. 1. After the FID signal 20 of the first .+-.90.degree. pulse has decayed, the refocussing is initiated by the first .+-.180.degree. pulse. A maximum focussing (maximal amplitude of the first spin echo 23) is reached after the time 2.tau. (calculated from the point in time of the first .+-.90.degree. pulse. The dephasing occurring thereafter is again reversed into a refocussing due to a further .+-.180.degree. pulse. A second spin echo 25 arises at point in time 4.tau. due to the refocussing, but this has a lower maximal amplitude since the chronologically irreversible spin-spin interactions arising in the interim cause a decrease of the magnetization in the X-Y plane. The next pulse sequence begins with a .+-.90.degree. pulse after a time T.sub.R.
The chronological curve of the first echo signal 23 is stored as S1(t) by the nuclear magnetic resonance apparatus and represents a line of a two-dimensional measuring matrix. Accordingly, the stored signal S2(t) of the second echo 25 likewise forms a line of a second two-dimensional measuring matrix. Other lines of the two measuring matrices are read in with every sequence given a corresponding selection of the gradient fields. In the case of image reconstruction according to method (b) above, each of the complete measuring matrices thus obtained is coverted into a result image by means of a two-dimensional Fourier transformation.
FIG. 2 shows a pulse sequence as employed in the "inversion recovery" method. This pulse sequence is identical to the spin-echo sequence except for a preceeding .+-.180.degree. pulse. In combination with the selection of a defined time difference I, this pulse effects a greater T.sub.1 emphasis of the images acquired in this way from the spin echoes 30 and 31.
So-called stimulated echoes occur upon employment of pulse sequences having three or more RF pulses, these stimulated echoes being caused by the Z-component of the magnetization present after the second 90.degree. pulse given a pulse sequence having three successive 90.degree. pulses.
Methods wherein stimulated echo signals are generated were initially described in the publication by E. L. Hahn, "Phys. Rev." 80 (1950), pp. 580-594. Since the information contained in the stimulated echo signal can also be evaluated for the imaging nuclear magnetic resonance, an object of the present invention is to generate this stimulated echo within a pulse sequence and to employ the echo information, for example, to produce an additional T.sub.1 -emphasized image.
Since the spin density .rho. and the relaxation times T.sub.1 and T.sub.2 are important parameters in MR imaging in tissue diagnosis, there is a need to obtain as much information as possible regarding these parameters in a short time. In conventional imaging systems, different T.sub.1 -emphasized and T.sub.2 -emphasized images could not be acquired in a sequence. The only possibility was to obtain the desired information in a plurality of measuring sequences.